For potential use in nuclear fusion reactors and for use in weapons systems, there is a need for convenient sources of relatively concentrated tritium. Tritium, which is a very minor isotopic component of hydrogen, is separable from lighter isotopes, but only by very tedious, expensive methods. An alternative to tritium isolation is tritium breeding in which other elements are transmutated to tritium through neutron capture. For example, tritium is produced by thermal neutron capture by .sup.6 Li which decays to tritium and helium. Nuclear reactors produce an excess of stray neutrons which might potentially be used in breeding tritium through neutron capture transmutation reactions. If a lithium-containing compound is disposed in the core of a nuclear reactor, tritium will be produced.
Particularly suitable lithium-containing compounds for tritium breeding are the lithium aluminum oxides, LiA1O.sub.2 and LiAl.sub.5 O.sub.8, which have high atom percents of lithium and have high melting points (respectively about 1610.degree. C. and 1900.degree. C.). Lithium aluminum oxide may be provided in the form of minute spherical particles as is taught in U.S. patent application, Ser. No. 339,697, filed Jan. 15, 1982, the teachings of which are incorporated herein by reference.
Tritium is a highly radioactive isotope and it presents particular difficulties in handling and containment because, like the other hydrogen isotopes, it has a tendency to diffuse through many materials. If a nuclear reactor is used to breed tritium, it is important to contain the bred tritium so that it does not contaminate the coolant gas or escape from the reactor environment. Thus, in a nuclear reactor, it is necessary to encase the breeding material in tritium-impermeable material. As one method of retaining tritium, particulate material, such as lithium aluminum oxide, may be coated with a tritium-impermeable shell. It has been proposed to coat lithium aluminum oxide particles with a TRISO type coating similar to that used for nuclear fuel particle coatings. This coating type consists of a layer of porous carbon, a layer of an isotropic dense carbon, a layer of silicon carbide and a layer of an isotropic dense carbon. For tritium breeding particles, the two most important layers are the porous carbon, the porosity of which supplies volume for accomodating the gaseous tritium and helium, and the silicon carbide, which is a barrier for the diffusive release of the tritium. However, difficulties have arisen when attempting to form such coated lithium aluminum oxide particles.
There are no problems in depositing the porous carbon layer at a temperature of about 1100.degree. C. or the isotropic dense carbon layer at a temperature of about 1300.degree. C. However, problems develop when depositing the silicon carbide layer at a temperature of about 1550.degree. C. At this temperature, lithium begins to be lost from the particle. Simultaneously, the inner dense carbon coating and the buffer coating often crack, and in extreme cases totally disintegrate, presumably due to the formulation of intercalation compounds between the lithium and the carbon. Thus, in the least damaging case, the particles contain little lithium after coating, and in the most damaging case, the particles break up during coating.
In order to effectively use coated lithium aluminum oxide particles for tritium breeding, it is necessary to develop a method of preventing lithium loss from the particles during silicon carbide coating.
It would be desirable to effectively coat lithium aluminum oxide with SiC in a manner that does not result in lithium loss therefrom.